The Test Structure and Composition of the Foraminifer Rosalina floridana*

SYNOPSIS. The composition of the test of Rosalina floridana (Cushman) was examined histochemically, and its structure was studied with the electron microscope by means of thin sections and carbon replicas. The test is composed of a thick organic lining overlain by one or more calcite layers bounded above and below by thin membranes. The membranes are fused to organic pore processes composed of coarse fibers that penetrate the calcite layers. The ***lining, consisting of coarse fibers matted into a laminated sheet, is considered a strengthening element of the test. The membranes covering each calcite layer are composed of fine, headed fibrils which in aggregate have a striated pattern; they are thought to be the crystal‐nucleating agent during calcification and to form a protective covering for the previously deposited calcite layers. The pore processes, which are devoid of an internal entrance for cytoplasm, are considered to be points of attachment for the membranes; they tie the organic test components into a unified whole. The calcite layers and the chambers lack this unity, being separated from each other and from the preceding chambers by membranes so that there are no calcite‐to‐calcite boundaries between them. An organic, sievelike structure of undetermined function has been found in the foramina of chambers near the prolocular region of the test. Histochemical methods show that the lining contains proteins, polysaccharides, and unidentified substances; the membranes and the pore processes stain as a protein‐polysaccharide complex free of other substances.